The emission of plasma waves by beams of electrons travelling in a plasma is a phenomenon of critical importance in applied plasma physics (for instance in problems directly related to the achievement of controlled nuclear fusion) and also astrophysical research (e.g. in the theory of solar radio bursts). In principle, the mechanisms involved are all contained in the Boltzmann-Vlasov equation, where the field is the self-consistent electromagnetic field produced by the interaction between beam and plasma. Unfortunately this celebrated equation cannot be solved directly, because both the analytical and numerical methods that can deal with this equation are plagued by secular terms which restrict the time domain of validity of the solutions to a few thousand plasma periods. In all applications of interest this domain is far too small; indeed in all astrophysical cases it is quite negligible compared with the duration of the observed phenomena (it is even much shorter than the time resolution of present-day equipment, such as dynamic spectrographs).

According to Dyson (1960), Malthusian pressures may have led extra-terrestrial civilizations to utilize significant fractions of the energy output from their stars or the total amount of matter in their planetary systems in their search for living space. This would have been achieved by constructing from a large number of independently orbiting colonies, an artificial biosphere surrounding their star. Biospheres of this nature are known as Dyson spheres. If enough matter is available to construct an optically thick Dyson sphere the result of such astroengineering activity, as far as observations from the earth are concerned, would be a point source of infra-red radiation which peaks in the 10 micron range. If not enough matter is available to completely block the stars’ light the result would be anomalous infra-red emission accompanying the visible radiation (Dyson 1960).

Although the gap between the observational data from northern and southern hemisphere started recently to narrow, the long neglected southern hemisphere is still far behind the northern. I have especially in mind binary and variable stars. The systematic research in this field in the south is not satisfactory and many interesting objects still await to be properly observed and many to be discovered. We are now grateful to our predecessors who 70 years ago started the photographic patorl of northern sky so that we are now able to trace back the behaviour of many interesting objects such as X-ray sources; we may follow back the changes of light curves and derive important conclusions from the changes of periods.